In the buildup to the World Cup, much of the focus is always on those unfortunate players who get injured in warm-up friendlies and are forced to miss the tournament. It is, indeed, in sporting terms, a tragedy. The event comes around once every four years, and for many players appearing in it, it is the highlight of a career or even a life. To have this taken away, maybe never to return, is cruel, indeed.

There is, though, a flip side of the coin -- those players who are drafted in to replace the fallen. They might have been organising their holidays, wondering where in the world they would be watching their national team and cheering them on. Instead, the dice have rolled in their favour, and they are taking part.

Perhaps the most striking example this time is Carlos Gruezo of Ecuador. Having just turned 19, the central midfielder was, in a quiet, undemonstrative way, one of the best players on show some 18 months ago in the South American Under-20 Championship. I picked him out for World Soccer magazine as one of the brightest hopes on show in the tournament. But that is what he was -- a hope. He is still young enough to play in next year's World Youth Cup. Ecuador may well have been thinking of making him the captain of that side -- or perhaps of drafting him into the squad for the 2015 Copa America in Chile -- as they seek to rebuild after Brazil and construct a side for the next set of World Cup qualifiers.

But it now seems likely that Gruezo will start the World Cup in the Ecuador team, standing to attention as the national anthem plays before the match against Switzerland in Brasilia on June 15.

First, his chances were boosted by a couple of injuries. Ecuador coach Reinaldo Rueda is a huge fan of Fernando Gaibor, a central midfielder who came up through the 2011 under-20 side. His place in the squad was guaranteed -- until he was injured. Alex Bolanos, tall and strong with an interesting range of passing, was another player who seemed set for a spot in the squad as one of the central midfield reserves. Rueda was watching his club form, and was impressed by the motivation he was showing. He, too, got injured. And so Gruezo was named in Ecuador's list of 30. Then he made the 23, and a place on the substitutes' bench became a possibility. But there was no way he would be picked ahead of the well-established duo of Segundo Castillo and Christian Noboa; the first the athletic midfield enforcer, the second the player whose passing skills knit the side together.

Tim Vickery is an English journalist who has been based in Brazil for the past 20 years. He is the South American football correspondent for the BBC Sport.

Last Saturday, in that horrific collision that has forced Mexico's Luis Montes out of the World Cup, Castillo suffered knee damage. For a while Castillo, too, seemed to be out of the tournament. The swelling came down, revealing that the injury was not as serious as originally feared. He was retained in the squad, although he is likely to be fit only in time for the third game. Gruezo has become his natural substitute and made his first international start on Wednesday against England.

Perhaps understandably, Gruezo looked nervous for the first few minutes. But once he settled in, he gave an accomplished display, showing the same virtues that had caught the eye at the under-20 level and doing the simple things well. His is a vital function. Ecuador's centre-backs are vulnerable and need plenty of protection. Noboa alongside him is not a natural defender -- and is also some way short of full fitness. So the responsibility for protecting the defence and playing the first ball forward will be given to this 19-year-old. It's a fascinating story.

Chile's Esteban Paredes is at the other end of his career. Approaching 34, the left-footed striker may have thought he had crowned his playing time by moving back to Chile earlier this year and skippering Colo-Colo to their 30th league title. He made a couple of appearances in South Africa four years ago, but a place in the squad for Brazil looked unlikely. He was named in the initial list of 30 but with little apparent chance of making the cut. Indeed, he was one of the first players to be dropped from the squad, with the Chilean FA issuing a statement thanking him for his services.

Esteban Paredes was thought to have played his last game for Chile.

Then attacking midfielder Matias Fernandez was injured, soon followed by Pablo Hernandez. Suddenly, Chile were looking a bit short in options in that position. They have Mauricio Pinilla as a target man centre-forward and Fabian Orellana as a reserve winger. The two leading strikers are Alexis Sanchez and Eduardo Vargas, with Jorge Valdivia as a talented playmaker -- but he, too, is injury-prone. And the mature Paredes does much of his best work outside the penalty area.

Perhaps the key moment came in the friendly against Egypt when Charles Aranguiz was pushed further forward into a position behind the strikers and looked uncomfortable in the role. He is better operating from a deeper position that allows him to show off his dynamism. Paredes, meanwhile, is more of a natural for the last 30 metres. And so, a few days after his country had dispensed with his services, Paredes is back in the squad and ready to round off his international career with a second World Cup.